In recent years, along with the progress of digital technology for electronic devices, large capacity, nonvolatile semiconductor storage devices are being actively developed for the purpose of storing data such as music, images, and information. For example, nonvolatile semiconductor storage devices in which ferroelectrics are used as capacitative elements are already used in many fields. Furthermore, in contrast to such nonvolatile semiconductor storage devices in which ferroelectric capacitors are used, variable resistance nonvolatile storage devices in which a material whose resistance value changes due to an application of an electric pulse, and maintains that state (hereinafter, also referred to as ReRAM) are gaining attention, since it is easy to achieve consistency with an ordinary semiconductor process.
As a variable resistance layer, a nickel oxide film (NiO), a vanadium oxide film (V2O5), a zinc oxide film (ZnO), a niobium oxide film (Nb2O5), a titanium oxide film (TiO2), a tungsten oxide film (WO3), a cobalt oxide film (CoO), or the like is used. It is known that such a transition metal oxide film exhibits a specific resistance value when a voltage or current greater than or equal to a threshold value is applied, and holds that resistance value until a voltage or current is newly applied, and furthermore has a feature of being able to be produced using the known DRAM process as it is.
Patent Literature (PTL) 1 discloses a configuration of a ReRAM having a cross-point structure, in which memory plugs are formed at crossing portions between electric conduction array lines in the X and Y directions. The memory plugs each include a variable resistance element and a diode element having a metal-insulator-metal (MIM) structure. The variable resistance element has a three-layered structure which includes a lower electrode layer, a composite metal oxide, and an upper electrode layer, and the diode element has a three-layered structure which includes a metal layer, an insulating layer, and a metal layer. An electrode layer which connects the variable resistance element and the diode element is further provided, and thus the memory plug has a stack structure which includes seven layers in total.
Further, PTL 2 discloses a cross-point type ReRAM which has, between a bit line and a word line, a resistance structure and a diode structure.
PTL 3 also discloses a cross-point ReRAM which has, between a bit line and a word line, a variable resistance element which includes a lower electrode, a variable resistor, and an upper electrode, and a nonlinear element connected to the variable resistance element in series.